1. Field of the Invention
The present invention relates generally to shock absorbers capable of responding to changes in fluid viscosity. More particularly, the present invention relates to a shock absorber that is temperature conscious and is thus responsive to changes in operating temperatures. The shock absorber of the present invention is accordingly highly sensitive to temperature variations and compensates for changes in oil viscosity throughout the normal range of ambient temperatures through the use of a valve system that incorporates a spring produced from a shape memory alloy.
2. Description of the Relevant Art
Shock absorbers are used in connection with automobile suspension systems and other vehicle suspension systems to absorb unwanted vibrations which occur during locomotion. To absorb this unwanted vibration, shock absorbers are generally connected between the body and the suspension of the automobile.
The most common type of shock absorber in automobiles is the dashpot type in which a piston is located within the shock absorber and is connected to the vehicle body through a piston rod. Because the piston is available to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed or extended, the shock vibration which would otherwise be transmitted from the suspension of the automobile to the body is absorbed by the shock absorber.
A conventional shock absorber comprises a pressure tube with a piston therein and a reserve tube surrounding the pressure tube. A piston rod connected to the piston projects from one end of the pressure tube. At the other end of the pressure tube is a valve communicating with the reserve tube. Damping is controlled by orifices in the piston which regulate passage of fluid from one side of the piston to the other.
The use of a viscous oil as the damping fluid causes the damping and rebound times of known shock absorbers to change at different temperatures. The oil is more viscous at cooler temperatures than at warmer temperatures. Accordingly, the fluid flow through the various orifices in the piston is slower at cold temperatures than at warm temperatures, thus resulting in less responsive damping and rebounding characteristics. This situation is problematic as heat is produced from the energy of the moving mass of the vehicle. While much of this heat is dissipated by radiation and conduction, the damping fluid itself absorbs some of it, thus becoming less viscous. Seasonal ambient air temperatures also have an impact on damping fluid viscosity.
Various attempts have been made over the years to eliminate or minimize the problem of temperature-influenced changes in the characteristics of the shock absorber. One approach is to allow for mechanical adjusting of the shock absorber, such as that embodied in U.S. Pat. No. 4,958,706, issued to Richardson et al. on Sep. 25, 1990, for ADJUSTABLE SHOCK ABSORBERS. The drawback with this mechanically adjustable approach is that the operator must physically make the necessary adjustments to compensate for the change in fluid temperature.
Other approaches included "automatic" responses built into the shock absorbers by relatively complex valve systems incorporated in the piston member itself. Examples of these approaches include U.S. Pat. No. 2,111,192, issued on Mar. 15, 1938 to Padgett for SHOCK ABSORBING MEANS, U.S. Pat. No. 3,107,752, issued on Oct. 22, 1963, to McLean for TEMPERATURE COMPENSATED VISCOUS DAMPER, and U.S. Pat. No. 4,785,921, issued on Nov. 22, 1988, to Hosan et al. for TEMPERATURE-COMPENSATING HYDRAULIC POSITIONER. While providing some level of "automation" thus eliminating dependence on the operator to make the correction, these designs are in generally overly-complex and do not provide direct temperature compensation.
In an effort to provide a mechanism which more directly responds to changes in temperature, U.S. Pat. No. 5,106,065, issued on Apr. 21, 1992, to Staton et al. for SELF-BLOCKING GAS SPRING WITH TEMPERATURE-RESPONSIVE BYPASS VALVE, provides a self-blocking gas spring having a temperature responsive bypass valve. A bimetallic disk is used to open and close a port depending on the temperature of the disk itself. If the temperature is low, the disk assumes a flat shape, closing the port and preventing the damping fluid from flowing through the bypass valve. If the temperature is high, the disk is warmed and assumes a curved shape which opens the port and allows the damping fluid to flow through the bypass valve. While having improved temperature-responsive characteristics, this system is relatively complex and requires extensive modification of known shock absorbers.
Accordingly, a shock absorber that is reliably and fully temperature responsive while utilizing a minimum number of components is wanting. SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a shock absorber in which changes in temperature do not affect compression or rebound characteristics.
Another object of the present invention is to provide a shock absorber that incorporates an auxiliary valve that opens as oil viscosity increases and closes as oil viscosity decreases.
An additional object of the present invention is to provide such a shock absorber in which the auxiliary valve is positionable between being fully opened and being fully closed and may be opened to any degree therebetween.
A further object of the present invention is to provide a valve having such a valve that utilizes a spring composed of a shape memory alloy.
A shock absorber according to the present invention comprises compression side to rebound side valves. In addition to these valves, an auxiliary valve is provided and includes an axial channel formed partially through the piston rod that terminates approximately in a side hole on the rebound side of the piston. Movably fitted within the channel is a sliding rod valve that is movable between an open position which allows passage of the damping fluid and a closed position which prohibits passage. The valve is tubular and includes a hole formed in its side at one end. A steel spring is positioned between the blind end of the channel and the sliding rod valve. A tubular sleeve is fitted in the channel on the compression side of the piston. Between the tubular sleeve and the rod valve is positioned a spring formed from a shape memory alloy.
At cooler temperatures the length of the shape memory alloy spring is contracted and the steel spring acts against the sliding rod valve to move the hole formed in its lower side in general alignment with the hole formed in the piston rod on the rebound side. As temperatures rise the length of the shape memory alloy spring increases, thus overcoming the resistive forces of the steel spring and pushing the slidable rod valve gradually toward the blind end of the channel so that the hole of the rod valve is gradually moved out of alignment with the hole in the side of the rod.
Thus the present invention provides a temperature-sensitive auxiliary flow path which allows damping fluid to bypass to varying degrees the normal rebound side to compression side flow of damping fluid as the fluid's viscosity changes.